Generally, it may be desirable to know properties of a gas flowing in a combustion system, such as in a boiler for steam generation in a power plant. In particular, it may be desirable to determine for instance the temperature of the gas in various locations in the combustion system. By determining the local temperature of the gas, it may be possible to control the gas flow or fuel distribution such that the gas temperature is optimally distributed as it flows through the combustion system. Thereby optimal combustion and minimal wear in the combustion system may be achieved.
One way to measure temperatures in a combustion system is by utilizing thermocouples mounted on the inside of the combustion chamber. However, in this way only the local gas temperature in one position will be measured for each thermocouple and the measured temperature is affected by radiation and therefore often deviates from the true gas temperature.
In order to obtain measurements of the gas temperatures in a cross-section of the gas flow in the boiler, acoustic methods have previously been utilized. To this end acoustic waves can be transmitted into the combustion system, whereby the temperature of the gas may be determined as a function of the time it takes for the acoustic wave to travel in the gas. This is due to the propagation speed of an acoustic wave being a function of the temperature of the gas. A plurality of transmitters may be distributed on the internal boiler wall in order to be able to obtain a two-dimensional image of the temperature distribution. However, high computational resources are needed for processing the measurements to be able to generate the two-dimensional image of the temperature distribution. For instance, it may be necessary to utilize interpolation in portions of the plane where no measurements have been carried out to be able to generate the temperature distribution image.
The concentration of individual gas molecules of a gas specie is another property of a gas that is desirable to know in a combustion system. By determining the gas concentration, the gas flow may be controlled so as to provide optimal concentration of the gas, whereby higher efficiency in regards of combustion in the combustion system may be achieved. More specifically, oxygen (O2) and other combustion gases such as carbon monoxide (CO) may be mixed uniformly such that the combustion becomes more efficient. Furthermore, the formation of NOx-gases is also reduced.
Suction pyrometric methods are known for determining a gas temperature in a combustion chamber. Suction pyrometry involves withdrawing gas from the combustion chamber, the gas passing a shielded thermocouple such that radiation effects are minimized and the true gas temperature is measured. With this technique temperatures up to 1100° C. may typically be measured. With more expensive materials, temperatures up to approximately 1600° C. can be measured. The extracted gas can thereafter be used also for determining the concentration of various gases in the combustion chamber. By positioning the probe at various locations in the combustion chamber, a spatial distribution of the concentration and temperature may be obtained. However, suction pyrometry does not provide for an efficient method to determine the spatial distribution of the concentration or temperature of the gas in the combustion system. In particular, it does not provide a real-time tool for determining the spatial distribution of the gas concentration and/or gas temperature.